Mass transfer has been well known and studied for many years. Examples include chemical separations, catalytic reactions wherein a species contacts a catalyst surface and exchanges mass with another species to form a compound, i.e. catalytic reaction. Exemplary apparati include kidney dialysis machines for separations wherein the mass transfer medium is a tube through which certain compounds pass because of a concentration gradient from the fluid within the tube to the fluid exterior to the tube. An example of a catalyzed mass transfer apparatus is a catalytic converter to reduce pollutants in automobile exhaust. Disadvantages of large scale mass transfer have been recognized and efforts made to use small scale mass transfer.
Separations and catalyzed reactions have been shown in microscale apparati as well. U.S. Pat. No. 5,534,328 to Ashmead et al. show a laminated structure wherein flow channels are made by etching a laminate partially through its thickness and stacking another laminate upon it to form a flow channel. Header holes through the laminate thickness are provided for inlets and outlets. Ashmead et al. suggest incorporating catalytic activity by packing a segment of a channel with catalytic beads or depositing catalytic materials onto the surface of a channel. Ashmead et al. further suggest mixer chambers formed by a half channel etched on the bottom of one laminate in combination with a half channel on the top of another laminate. A disadvantage of the construction of Ashmead et al. is the complexity and expense of carving laminates partially through the thickness of the laminates. A further disadvantage of the construction of Ashmead et al. is the small aspect ratio of width to depth of their channels for flow resistance and pressure drop. The construction of Ashmead et al. cannot achieve diffusive mass transfer, or controlled mixing by actuation.
Thus, there remains a need for a microchannel mass exchanger having a lower cost of fabrication and which provides a reduced pressure drop.